The importance of secondary batteries such as lithium secondary batteries or nickel hydrogen batteries has grown in recent years as vehicle-mounted power supplies used in vehicles powered by electricity and as power supplies installed in personal computers, portable terminals and other electrical products. In particular, lithium secondary batteries are expected to be preferably used as high-output, vehicle-mounted power supplies due to their light weight and high energy density.
A typical configuration of this type of lithium secondary battery has a positive electrode active material layer (and more specifically, a positive electrode active material layer and negative electrode active material layer) on a surface of a positive electrode collector that is capable of reversibly occluding and releasing lithium ions. For example, in the case of a positive electrode, the positive electrode active material layer is formed by coating onto the positive electrode current collector a paste-like composition for forming a positive electrode active material layer (including a slurry-like composition or ink-like composition, and to be simply referred to as an “electrode paste”) that is prepared by mixing a positive electrode active material such as a lithium transition complex oxide with a powder of a highly electrically conductive material (electrically conductive material) and a binder and the like in a suitable solvent.
Here, in the case the electrode paste is prepared using an organic solvent for the solvent, a water-insoluble polymer that is insoluble in an organic solvent such as polyvinylidene fluoride (PVDF) is used for the binder. On the other hand, in the case the electrode paste is prepared using an aqueous solvent for the solvent, a water-soluble polymer or water-dispersible polymer such as polytetrafluoroethylene (PTFE) or carboxymethyl cellulose (CMC) is preferably used for the binder. Among these, an electrode paste that uses the latter aqueous solvent (to be referred to as an “aqueous electrode paste”) offers the advantage of reducing the overall environmental burden since it requires a smaller amount of organic solvent and other industrial waste while also not resulting in corresponding equipment and processing costs.
However, the aqueous electrode paste described above tends to cause an increase in pH attributable to its high reactivity with water depending on the contents of the positive electrode active material (such as in the case of a lithium-nickel-based complex oxide having a composition represented by the formula: LiNiO2). When this aqueous electrode paste having a high pH is coated onto a metal positive electrode current collector (such as that made of aluminum), a compound demonstrating high electrical resistance (such as an oxide or hydroxide) is formed on the surface of the current collector, thereby potentially causing corrosion of the positive electrode current collector and an increase in internal resistance of the battery.
Patent Document 1 is an example of the prior art relating to a positive electrode of a lithium secondary battery that uses this type of aqueous electrode paste. According to the technology described in Patent Document 1, by interposing an electrically conductive layer containing an electrically conductive material formed with a non-aqueous electrode paste between a positive electrode current collector and an active material layer formed with an aqueous electrode paste, formation of compounds having high electrical resistance that cause corrosion of the positive electrode current collector is prevented.    Patent Document 1: Japanese Patent Application Laid-open No. 2006-4739